Activated carbon because of its adsorptive qualities is used in many industries. One example is water treatment where water is filtered through activated carbon. It is also used for extraction of gold and other precious metals from solutions. Because of the high cost of activated carbon, it is reused if and when possible by being reactivated. This is especially true in the precious metals industry because of the large quantities of activated carbon used and the rapid fouling of the carbon particles that occurs. In order to reactivate carbon it must be heated up to a temperature between about 600.degree. C. and 800.degree. C. for a period of time sufficient to volatilize the fouling agents and open the pores and active sites on the carbon particles. The carbon particles are typically granular, either naturally occurring or extruded. Particle size is generally in the range of 6.times.24 mesh. The reactivation process is generally carried out in an indirectly heated kiln.
In existing reactivation processes, carbon particles are conveyed to the kiln by pumping or educting and dewatered through a dewatering screen. The carbon particles are generally conveyed at a higher rate than the kiln's production rate, thus the dewatered carbon particles are held in a surge bin. In most cases the dewatered carbon particles have a moisture content of between about 40% and 50% wet basis as metered to the kiln.
There are many different kiln designs available but the most successful and reliable kiln has been the horizontal indirectly heated kiln. The two most common versions of this kiln are fossil fuel (which includes but is not limited to No. 2 oil, propane gas or natural gas) fired kilns and electrically heated kilns.
The major differences between the fossil fuel and the electric kiln are the sources of energy and their energy efficiencies. For a given size of kiln, each produces the same quantity and quality of product.
The most energy efficient kiln is the electric kiln which transfers its heat to the process through radiation and free convection. There is no combustion taking place in the furnace and there are no exhaust stacks, although the volatile gases produced by heating must be vented. The only energy losses are those from the kiln shell surface, the furnace surface losses, and electrical conductor losses. All of these losses are small and the overall efficiency of the process can be in excess of 75%. The kiln shell surface losses are required for product cooling and these losses together with the electrical conductor losses may be controlled through good design.
The fossil fuel kiln has the same energy losses as the electric kiln except for the conductor losses. The fossil fuel kiln however has a significant energy loss due to combustion products which must be expelled from the furnace. As the operating temperature in the furnace increases, the efficiency of the furnace decreases. If the process temperature is 700.degree. C., then the products of combustion must be at this temperature or higher. The high volume of high temperature gas reduces the efficiency of the fossil fuel kiln to between 30% and 40% in the reactivation temperature range depending on the air to fuel ratio.
In order to increase the energy efficiency of the fossil fuel kiln, the heat from the kiln furnace flue must be recovered. Recovery methods include preheating the combustion air by means of a heat exchanger to remove the heat from the flues. In practical and economic terms, the preheating of combustion air has a temperature limit. This may bring the overall efficiency to about 50%.
Other methods include directing the furnace flue gases through the kiln itself in a counterflow arrangement. The counterflow approach has both technical and process problems associated with it since the temperature of the fouling agents is continually dropping which can result in condensation of these agents back into the carbon.